386 POPULAR SCIENCE MONTHLY, 



ley, the curtain is once more drawn, as it were, over the modern 

 world. 



At Lac h la Tortue, twenty-one miles from Three Rivers, we 

 are again in the dim past. We can stand upon the shore of this 

 lake and see a sight that might have been the o-iginal for an 

 illustration of iron-ore gathering in Scandinavia one hundred 

 years ago. 



This Lac k la Tortue (Turtle Lake) is our curious Canadian 

 iron mine. It is a body of water about four miles long by a 

 mile and a quarter in average width, occupying the center of a 

 large area of swampy land. The surrounding land is largely com- 

 posed of sand formed by the wearing down of the Archaean rocks 

 by glacial action. 



It is well known that decaying vegetable matter yields acids 

 that dissolve the oxide of iron. Evidences of this solvent action 

 of vegetable acids on iron are frequently seen in pieces of slate. 

 The slate is colored by iron, but frequently white or light- colored 

 spots occur. These are points where a leaf or a fragment of bark 

 has been deposited with the fine mud in which form the slate was 

 deposited. The leaf or bark has decayed, the vegetable acids thus 

 formed have dissolved the iron oxide to which the color of the slate 

 was due, and of course a white or colorless patch is formed. 



In the sandy area around Lac a la Tortue we find the most 

 favorable conditions for the action of vegetable acids on iron 

 oxide. The sandy land produces a rank vegetation, and its decay 

 furnishes abundance of organic acids. These acids are in solution 

 in the drainage waters, which on their way to the lake percolate 

 through the sand. They thus come into contact with the iron 

 oxide in the finely divided materials, dissolve it, and carry it along 

 to the lake. Here a new chemical action comes into play. The 

 solution of iron in vegetable acid (in which the iron is in what the 

 chemist calls the form of a protosalt) is oxidized by the action of 

 the air on the surface of the lake into a persalt, which is insoluble, 

 and appears on the surface in patches that display the peculiar 

 iridescence characteristic of petroleum floating on water. Indeed, 

 not infrequently these films of peroxide of iron are incorrectly 

 attributed to petroleum. These films become heavy by addition 

 of new particles, they sink through the water, and in this manner, 

 in time, a large amount of the iron ore is deposited on the lake bot- 

 tom. It must not be supposed that the ore is deposited as a fine 

 mud or sediment. On the contrary, in this lake ore, as it is called, 

 we have an excellent illustration of what is known as concretion- 

 ary action that is, the tendency of matter when in a fine state of 

 division to aggregate its particles into masses about some central 

 nucleus, which may be a fragment of sunken wood, a grain of 

 sand, or indeed a preformed small mass of itself. Precipitated in 



